This invention concerns circular knitting and hosiery machines in general and refers in particular to a device for varying the stitch length on said machines.
As it is well known, circular knitting and hosiery machines basically comprise:
a rotating cylinder with a cylinder-support having a plurality of longitudinal grooves in which are housed and move vertical needles governed by rising and lowering cams positioned on a hood around the cylinder.
a sinker bed placed around the top of the cylinder, turning with the same and holding a number of horizontal sinkers which radially alternate with the vertical needles on the cylinder and which together form a formation plane of the stitches on the basis of the rising and lowering of the needles, and
a possible dial bed placed coaxially above the cylinder and housing other fixtures, such as hooks or horizontal needles used in forming different types of stitches.
These machines are also equipped, among other things, with means for varying the lengths of the stitches according to needs.
According to a known technique, one of the known ways for varying the length of the stitches is to move and position the height of the cylinder in relation to the cylinder support and stitch formation plane defined by the sinkers, on the sinker bed changing by this means the level of the needles in respect to the sinkers.
This methods however, involves for the presence of vertical guides between the cylinder support and cylinder and moving the whole cylinder mass. The cylinder support must in this case have size sufficient to easily support the cylinder in all its positions and precision work requirements so as to avoid out of center coupling with the cylinder. Furthermore, the change in height level of the cylinder requires a lifting group capable of developing a relatively strong force, compatible with the cylinder mass itself.
Then, as is well known, the needles are controlled by selection groups through jacks working in conjunction with them. The jacks move upwards in unison with the upward movements of the cylinder, with the result that they vary their level compared with the selection groups, which remain at a fixed height, and jeopardize the selection system. At present, to avoid this drawback, means for maintaining a fixed level are associated with the jacks which, notwithstanding the upward movements of the cylinder, work in conjunction with the selection groups. These means however imply further complications and added costs.
One object of the invention is to remove the drawbacks and disadvantages of the known technique and consequently be able to vary the stitch length of circular hosiery and knitting machines, without having to move the rotating cylinder mass.
Another object of the invention is to provide a device which enables the variation in the stitch length to be carried out on the machines in question by moving upwards a less cumbersome organ, improving at the same time the coupling conditions between the cylinder support and the cylinder.
These objectives are achieved, in accordance with the invention, on the one hand by keeping the height of the cylinder with the needles fixed and, on the other, by changing the height of the sinker bed in respect to the cylinder so as to vary the level of the stitch formation plane defined by said sinkers.
So, and advantageously, the cylinder can be, and remain, physically axed to the cylinder support, a condition which reduces the out of center problems of coupling between two units and in the rotation of the cylinder. Then, since the cylinder remains at a fixed height, the jacks no longer require any means to hold then always at the same level as the selection groups, with obvious advantages at least from an economic viewpoint. In addition, by remaining at a fixed height, the cylinder no longer requires vertical guides, the cylinder support implies less mechanical workings, and be at a more limited height, therefore lighter, without however compromising the rigidity of the two coupled units. Furthermore, the changes in height of only the sinkers certainly requires less driving force drive as would otherwise be necessary to change the height of all the cylinder and consequently a simpler and more economic lifting units.